On Radon measures invariant under horospherical flows on geometrically infinite quotients

Abstract

We consider a locally finite (Radon) measure on $S{O}^{+}(d,1)/\Gamma$ invariant under a horospherical subgroup of $S{O}^{+}(d,1)$ where $\Gamma$ is a discrete, but not necessarily geometrically finite, subgroup. We show that whenever the measure does not observe any additional invariance properties then it must be supported on a set of points with geometrically degenerate trajectories under the corresponding contracting $1$-parameter diagonalizable flow (geodesic flow).

28 pages, 4 figures, results unchanged, minor revisions including a simplified proof of Corollary 1.4